Welding power supply apparatus generally require a number of heat generating electrical apparatus such as transformers, power modules, inductors, rectifiers, transducers and the like interconnected through circuit boards. A fan is generally provided to circulate air throughout the structure to cool the heat generating apparatus. However air circulated through the housing includes dust which adversely affects the operation of the electrical system and more particularly, the printed circuit boards. In order to protect the circuit boards from contamination, plastic covers are often used to separately enclose the circuit boards.